Hydrocarbon stream feeds like pyrolysis gas feeds can have diene values ranging from 1-120 and diolefin weight percentages in such streams can range from 0.5 weight percent to 50 weight percent or above. Under an oxygen atmosphere diolefins are unstable. Diolefins present a challenge for processes involving catalysts because the diolefins are very reactive and polymerize even under hydrogen atmospheres at high temperatures forming gum. Because of the reactivity of diolefins catalysts that have poor activity are restricted in cycle length and have a propensity for polymerization because of the requirement for high temperatures. It is generally accepted at temperatures about 170° C. (338° F.) excess polymerization causes pressure drops across a catalytic reactor. These problems are generally worse if the catalyst comprises a porous active base such as gamma or theta alumina where polymerization of diolefins can cause swelling of the porous catalyst and can damage the structure of the catalyst.
In situations where the catalyst is an active catalyst there is a tendency for the active catalysts to convert diolefins and acetylenes as well as the monoolefins rapidly and often more selectively to their corresponding paraffins and naphthenes causing excess heat generation. Again, these conditions tend to favor gum formation and this is particularly so in a commercial application where a fixed bed adiabatic reactor is subjected to high temperature rises. The reactor's practical operating window is limited because of the pressure drop problems.
The current industrial practice for selectively hydrogenating diolefins or unsaturated hydrocarbon fractions is based on the use of sulfided nickel catalysts operating at moderately high temperatures of approximately 185° C. (365° F.). Sulfur loss from the catalyst to the product occurs and sulfur must be replenished to keep the catalyst active and operating optimally. Furthermore, once the sulfur is lost into the product, in some instances the sulfur must also be removed from the product and this adds another level of processing.
Other types of selective hydrogenation processes are also known, such as that described in JP54157507A. JP 54157507A describes the use of a palladium catalyst on an alumina support to selectively hydrogenate acetylene and methyl acetylene (alkynes) that are present in olefin fractions obtained in petrochemical processes. The catalyst described in JP54157507A comprises a thin alumina coating over an alpha alumina carrier of spherical or cylindrical shape and being around 1-20 mm in size, length and diameter. The alumina precursor, which can be aluminum nitrate, aluminum chloride, aluminum hydroxide and the like, is coated onto the alpha alumina carrier and then the coated alpha alumina carrier and alumina precursor is heat treated at between 400° C. (752° F.) to 700° C. (1292° F.) to create a thin alumina coating over the alpha alumina carrier. A palladium compound such as palladium chloride, palladium nitrate, and the like is dissolved in a suitable solvent, and then applied to the alumina coating to give effectively an enriched surface coating containing palladium. JP54157507A describes the use of the resulting catalyst in the selective hydrogenation of acetylene in a composition comprising ethylene.
US 2003/0036476 A1 describes a coated catalyst having a core and a shell surrounding the core, the core is made up of an inert support material. The shell is made up of a porous support substance, and the shell is physically attached to the core. A catalytically active metal selected from the group consisting of the metals of the 10th and 11th groups of the Periodic Table of the Elements is present in finely divided form in the shell. The coated catalyst is described as being suitable for the selective reduction of unsaturated hydrocarbons, specifically lower C2-C4 unsaturated hydrocarbons.
U.S. Pat. No. 6,177,381 B1, which is incorporated by reference in its entirety, describes a layered catalyst composition showing improved durability and selectivity for dehydrogenating hydrocarbons, a process for preparing the catalyst and processes for using the composition. The catalyst composition comprises an inner core such as alpha-alumina, and an outer layer bonded to the inner core composed of an outer non-refractory inorganic oxide such as gamma-alumina. The outer layer has uniformly dispersed thereon a platinum group metal such as platinum and a promoter metal such as tin. The composition also contains a modifier metal such as lithium. The catalyst composition is prepared by using an organic binding agent such as polyvinyl alcohol which increases the bond between the outer layer and the inner core. The catalyst composition is described as also being suitable for use in dehydrogenation and hydrogenation processes. Likewise, U.S. Pat. No. 6,280,608 B1 also describes a layered catalyst suitable for use in dehydrogenation and hydrogenation processes, while U.S. Pat. No. 6,486,370 B1 is directed to a layered catalyst suitable for use in dehydrogenation processes.
US 2006/0266673 A1 and US 2006/0270865 A1 describe a similar layered catalyst, but with an additional fibrous component in the outer layer. The fiber-containing layered catalyst is described as being suitable for use in dehydrogenation and hydrogenation processes including selective hydrogenation of dienes and trienes.
Improvements in selective hydrogenation is very important for reducing waste and recycle, thereby saving energy and material in the process. Improvements have a significant economic impact on the production of olefins.